The worldwide need for clean and sustainable energy is constantly increasing and electrochemical devices, such as batteries, fuel cells, and dye-sensitized photovoltaic cells, are among the most promising sources. (B. Smitha, S. Sridhar, A. A. Khan, Journal of Membrane Science 2005, 259, 10; G. Wegner, Polymers for Advanced Technologies 2006, 17, 705; R. G. Rajendran, MRS Bulletin 2005, 30, 587; and W. Vielstich, A. Lamm, H. A. Gasteiger, Handbook Of Fuel Cells: Fundamentals, Technology, and Applications, Wiley, Chichester, England; New York, 2003.) At the core of these devices is an electrolyte which facilitates charge transport between electrodes. Commonly used liquid or gel electrolytes preclude widespread use of these devices due to processing difficulties and safety concerns. However, polymer ionic conductors offer high mechanical strength and greater fabrication flexibility compared to traditional electrolytes, as well as better physical separation of electrodes. Polymer electrolytes are generally thin films that facilitate the transport of a given ion or ions at predetermined operating conditions. Although the desired properties of solid polymer electrolytes depend on the device application, fast ion conduction is essential to reduce electrical resistance.
Layer-by-layer (LBL) assembly is a versatile thin-film fabrication technique which consists of the repeated, sequential immersion of a substrate into aqueous solutions of complementary functionalized materials. (G. Decher, Science 1997, 277, 1232.) Utilizing electrostatic forces or secondary interactions, such as hydrogen bonding, LBL processing provides nanoscale blending of polymers and other organic/inorganic materials which are otherwise impossible to construct. The composition, morphology, and bulk properties are controlled by adjusting assembly parameters, such as pH and ionic strength. This technique has been adapted to many other platforms, such as spraying, spin-assisted assembly, and roll-to-roll processing. (J. Cho, K. Char, J. D. Hong, K. B. Lee, Advanced Materials 2001, 13, 1076.) The high versatility, tunability, and ease of processing from the ability to use aqueous solutions make this system a great competitor to traditionally assembled solid state conductors. Previously, LBL assembled systems have shown promise as thin film conductors for photovoltaics, electrochromic devices, and fuel cells, but they were limited in scope of application due to low ion conductivity values. (G. M. Lowman, P. T. Hammond, Small 2005, 1, 1070; D. M. DeLongchamp, M. Kastantin, P. T. Hammond, Chemistry of Materials 2003, 15, 1575; T. R. Farhat, P. T. Hammond, Advanced Functional Materials 2005, 15, 945.) To illustrate, the highest conductivity values achieved in an LBL film to date have been on the order of 10−5 S/cm, while typical values for fully hydrated LBL films are in the 10−7 to 10−9 S/cm range. (D. M. DeLongchamp, P. T. Hammond, Langmuir 2004, 20, 5403; and D. M. DeLongchamp, P. T. Hammond, Chemistry of Materials 2003, 15, 1165.)
Because the LBL films can be tailored to deposit any polyelectrolyte (PE) couple to any desired thickness, ranging from a few angstroms to a few microns, they are much less expensive technology than conventional membranes. Ion permeability and ion conductivity in LBL films have been extensively studied and characterized. (Krasemann, L.; Tieke, B. Langmuir 2000, 16, 287; Farhat, T. R.; Schlenoff, J. B. Langmuir 2001, 17, 1184-1192; Farhat, T. R.; Schlenoff, J. B. J. Am. Chem. Soc. 2003, 125, 4627-4636; Harris, J. J.; DeRose, P. M.; Bruening, M. L. J. Am. Chem. Soc. 1999, 121, 1978; DeLongchamp, D. M.; Hammond, P. T. Chem. Mater. 2003, 15, 1165-1173; and DeLongchamp, D. M.; Hammond, P. T. Abstr. Pap. Am. Chem. Soc. S22:136-PMSE, Part 2 2001.) The diffusion coefficient of ions of conventional polymer multilayers is a few orders of magnitude lower than the classical ion exchanger membranes hence their proton conduction is lower. However, a range of multilayer systems which incorporate hydrophilic polymers using electrostatic and hydrogen bonding mechanisms, and have shown increases in ionic conductivity of three or four orders of magnitude. (DeLongchamp, D. M.; Hammond, P. T. Chem. Mater. 2003, 15, 1165-1173; DeLongchamp, D. M.; Hammond, P. T. Abstr. Pap. Am. Chem. Soc. S22:136-PMSE, Part 2 2001; and Tokuhisa, H.; Hammond, P. T. Adv. Funct. Mater. 2003, 13, 831-839.) These differences are further enhanced by the fact that ultra thin films can be formed using the LBL technique, making the final conductance closer to that required for power applications. One can tune the thickness and permeability, as well as the composition, of these films through choice of polyelectrolytes and adsorption conditions. For example, using strong polyelectrolytes with hydrocarbon backbones yields LBL films that tend to be either strongly or moderately hydrophobic, thus discouraging proton exchange. On the other hand, LBL films assembled using weak electrostatic and secondary interactions (i.e., long-range hydrogen bonding or dipole-dipole), particularly those with hydrophilic backbones, support proton-exchange.
There remains a need for improved ion-conducting materials for electrochemical device applications for devices such as, for example, proton-exchange membrane fuel cells, electrochemical splitting of water, photovoltaic cells, sensors, dye-sensitized photovoltaic cells, light-emitting electrochemical cells, batteries, and electrochromic devices.